The present invention relates to an image projection apparatus for enlarging and projecting an image received and displayed by a picture tube, and in particular to such an image projection apparatus which is excellent in the power efficiency and luminous flux utilization efficiency.
FIG. 1 illustrates the general configuration of an image projection apparatus. In FIG. 1, reference numeral 1 denotes a faceplate of a CRT (cathode ray tube). Numerals 2 and 3 denote a lens and a screen, respectively. In general, a compound lens composed of several lenses is used as the lens 2. In FIG 1, however, the compound lens is represented by a single lens.
In the configuration illustrated in FIG. 1, the surface illuminance L.sub.s (lux) of the screen 3 is related to the luminance Bt (cd/m.sup.2) of the CRT faceplate 1 by EQU M.sup.2 L.sub.s =.eta..multidot..pi..multidot.B.sub.t .multidot.sin.sup.2 .theta..sub.M, (1)
where:
.eta.: light transmittance of the lens 2 PA1 .theta..sub.M : half of angular aperture illustrated in FIG. 1 PA1 M: magnification PA1 .pi.: pi PA1 F=0.9 PA1 M.apprxeq.1.0 PA1 sin.sup.2 .theta..sub.M =0.20 (.theta..sub.M =27.degree.) PA1 .alpha.=25.degree. (half of field angle illustrated in FIG. 1) PA1 .eta.=0.7.
The expression (1) is obtained on the assumption that the faceplate 1 of the CRT forms a uniform diffusion surface and the so-called Lambert's cosine law holds true with respect to the light radiant intensity of the faceplate 1.
FIG. 2 illustrates the above described cosine law. Assuming that the light radiant intensity in the direction of the normal line of an area element taken on the plane 1 is B.sub.t, it is said that the cosine law holds true with respect to the radiant intensity of the plane 1 provided that the radiant intensity in a direction forming an angle of .theta. with respect to the normal line is B.sub.t cos .theta.. This is called the Lambert's cosine law.
FIG. 3 illustrates the relationship between the solid angle element d.OMEGA. and the plane angle element d.theta.. The solid angle element d.OMEGA. is generally given by EQU d.OMEGA.=2.pi. sin .theta..multidot.d.theta..
From the foregoing description, it is understood that the quantity of light L captured by the lens 2 illustrated in FIG. 1, which is a part of the quantity of light emitted from a unit area (area element) of the faceplate 1, is given by ##EQU1##
Therefore, the right side of the expression (1) is equal to .theta.L. This value is equal to the quantity of light projected from a unit area on the faceplate 1 toward M.sup.2 units of area on the screen 3 through the lens 2. That is to say, this value is equal to the left side of the expression (1), M.sup.2 L.sub.s.
From the above description, it is understood that sin.sup.2 .theta..sub.M indicates the converging power of the lens 2. The value of sin .theta..sub.M is related to the F value (the ratio of the focal length to the aperture) of a lens by ##EQU2##
In recent years, various improvements have been vigorously introduced in a lens used for image projection. The limit values available in practical use at the present stage are as follows:
That is to say, the converging power is as low as approximately 20%.
Referring to FIG. 1, this means that the quantity of light as much as approximately 80% of the total amount of light emitted from the faceplate 1 has been discarded as the light which is outside of the half of the angular aperture .theta..sub.M (=27.degree.). It has been keenly desired to hold the value of the half of a field angle .alpha. to be above 25.degree. and to increase the converging power while maintaining the favorable focusing. Due to the limit in the lens design technology, however, it has heretofore been thought that it is extremely difficult to raise the converging power to be above 20%.
As described later, therefore, the inventor having studied causes of decrease in the converging power consider that the fundamental cause is that the light is emitted from the CRT faceplate over a wide angle range because of a uniform diffusion plane of the CRT faceplate. Accordingly, the inventor will propose to narrow the emission angle of the light emitted from the faceplate. Since such a proposal itself is already known, this known art will now be described.
FIG. 4 illustrates an conventional image projection apparatus which has been proposed in the U.S. Pat. No. 2,093,288, "Television Apparatus". In FIG. 4 numeral 1 denotes a CRT faceplate and numeral 5 denotes an exit plane of the light. The space between the CRT faceplate 1 and the transmission plane 5 is filled with a medium.
The conventional image projection apparatus illustrated in FIG. 4 aims at narrowing the divergence angle of the emitted light at the faceplate 1. As described later in detail, however, a CRT having a large emission angle (divergence angle) of the light emitted from the faceplate 1 is necessarily presupposed because of the structure of the conventional faceplate of CRT (picture tube). Accordingly, the conventional image projection apparatus illustrated in FIG. 4 also has the drawback that the luminous flux utilization efficiency is poor.
In addition, since the faceplate 1 must be formed of a flat plane, the spherical aberration for the focused image may be eliminated only on the optical axis. For the periphery of the image plane, however, large amount of astigmatism and coma are produced. That is to say, when beams a, b, and c are traced backward in FIG. 4, they do not converge to a point. And the occurrence of the spherical aberration at a point e is perceived. It will result in the deteriorated focusing of the projected image, bringing about a serious drawback.